This invention relates to communications systems and methods, and in particular to radiotelephone communications systems and methods.
Cellular radiotelephone systems are commonly employed to provide voice and data communications to a plurality of subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as designated IS-54B in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993.
FIG. 1 illustrates a conventional terrestrial cellular telephone communication system 100. The cellular radiotelephone communication system 100 includes one or more radiotelephones 102 communicating with a plurality of cells 104, each of which is served by a base station (BS) 106a-106n. The base stations 106a-106n communicate with a mobile telephone switching office (MTSO) 108. Although only six cells are shown in FIG. 1, a typical cellular network may comprise hundreds of cells, may include more than one MTSO and may serve thousands of radiotelephones.
The cells 104 generally serve as nodes in the communication system 100 from which links are established between radiotelephones 102 and the MTSO 108 by way of the base stations 106a-106n serving the cells 104. Each cell will have allocated to it one or more dedicated control channels 110a-110n, and one or more traffic channels 112a-112n. The radiotelephone traffic channels are used by the base stations for radiotelephone communications (such as voice and/or data communications) with the radiotelephones. The control channels are used by the base stations to control radiotelephone communications with the radiotelephones over the radiotelephone traffic channels. Thus, for example, the control channel may be used to broadcast system information to the radiotelephones and also to send and receive access messages to and from individual radiotelephones. The base stations 106a-106n are connected to the MTSO by a plurality of bidirectional links 114a-114n, which may be hardwired or wireless links. Through the cellular network 100, a duplex radio communication link may be effected between two radiotelephones 102 or between a radiotelephone 102 and a land line telephone user. The base stations 106a-106n handle radio communication between the radiotelephones in the associated cell 104 and the MTSO 108. In this capacity, the base stations 106a-106n function chiefly as a relay station for data and voice signals.
As shown in FIG. 1, the dedicated control channels 110a-100n are generally broadcast using a different radio frequency F1-Fn per cell. The traffic channels 112a-112n are allocated among a plurality of radio frequencies to reduce and preferably eliminate interference using conventional frequency allocation techniques that are well known to those having skill in the art. As also shown in FIG. 1, several cells may be combined into larger groups 120, called paging areas in cellular radiotelephone standard IS-136 or location areas in GSM. The grouped paging areas can reduce the paging and registration loads on the system. Pages to radiotelephones may only be transmitted to the paging area 120 in which the radiotelephone is located. Registration need only take place when a radiotelephone moves between paging areas.
Unfortunately, a cellular radiotelephone system 100 as described in FIG. 1 may have excess control channel capacity in each cell. More specifically, each control channel may have the capacity to control many more radiotelephones than are generally found within the cell. Accordingly, valuable radio spectrum may be wasted.
This wasted control channel radio spectrum may be reduced by distributing a common control channel via many base stations, forming a larger logical cell including many smaller cells around each base station. The common control channel is multicast on the same frequency in every base station. Accordingly, as shown in FIG. 2, cellular radiotelephone system 200 includes radiotelephones 102, cells 104, MTSO 108, traffic channels 112a-112n and links 114a-114n, as was already described in connection with FIG. 1. However, in contrast with FIG. 1, each base station 206a-206n broadcasts the same control channel 210 on the same frequency F1. Thus, a common control channel is transmitted by the base stations to control radiotelephone communications with the radiotelephones over the radiotelephone traffic channels. By using a common control channel on a common frequency F1, excess capacity in the control channel may be reduced.
Unfortunately, it may be difficult to find a frequency F1 that can be used by the common control channel of all the base stations without interference. Interference may occur in conventional cellular radiotelephone systems, as described in connection with FIGS. 1 and 2. Moreover, interference may be exacerbated in Wireless Office Systems (WOS) that are installed in a building to provide cellular radiotelephone communication among cellular radiotelephones that are located in the building. When installing a wireless office system in a building, there may be excess capacity in the control channel. For example, a wireless office system may be designed for installation in offices with up to 450 users. However, the control channel may have a capacity of up to 1000 users. Thus, a common control channel may be preferred to reduce excess capacity. However, it may be difficult to identify a radio frequency that can be used by all of the WOS base stations to transmit the common control channel.
It is therefore an object of the present invention to provide improved cellular radiotelephone systems and methods.
It is another object of the present invention to provide cellular radiotelephone systems and methods that can reduce excess capacity in the control channel that is transmitted by the base stations.
It is yet another object of the present invention to provide cellular radiotelephone systems and methods that can reduce the excess capacity of the control channel without incurring excessive control channel interference.
These and other objects are provided, according to the present invention, by transmitting a common control channel in a cellular radiotelephone system from a first one of the base stations over a first radio frequency and from a second one of the base stations over a second radio frequency that is different from the first radio frequency. By providing a common control channel, excess control channel capacity can be reduced and preferably eliminated, thereby conserving control channel radio spectrum. Moreover, by transmitting the common control channel using multiple radio frequencies, the radio frequencies can be selected to reduce and preferably eliminate interference.
More specifically, cellular radiotelephone systems and methods according to the invention communicate with a plurality of cellular radiotelephones via a plurality of base stations. A plurality of radiotelephone traffic channels are used by the base stations for radiotelephone communications with the radiotelephones. A common control channel is transmitted by the base stations to control radiotelephone communications with the radiotelephones over the radiotelephone traffic channels. According to the invention, the common control channel is transmitted by a first one of the base stations over a first radio frequency and is transmitted by a second one of the base stations over a second radio frequency that is different from the first radio frequency. The cellular radiotelephone system also preferably comprises a central control channel handler (CCH) that synchronously transmits the common control channel to the plurality of base stations.
In a preferred embodiment of the present invention, the plurality of base stations are Wireless Office Systems (WOS) base stations that are located in a building to provide cellular radiotelephone communication among cellular radiotelephones that are located in the building. The common control channel is transmitted by the base stations in the building to control radiotelephone communications with the radiotelephones in the building over the radiotelephone traffic channels. The common control channel is transmitted by the first one of the base stations in the building over a first radio frequency and is transmitted by a second one of the base stations in the building over a second radio frequency that is different from the first radio frequency.
A first group of base stations may transmit the common control channel over the first frequency, and a second group of base stations may transmit the common control channel over the second frequency. In a wireless office system as described above, the first group of base stations may be located in a first portion of a building, and the second group of base stations may be located in a second portion of the building. Accordingly, a radio frequency for groups of base stations in a building may be selected to reduce and preferably eliminate interference with the cellular system outside the building, while at the same time broadcasting a common control channel over all of the base stations, to reduce excess control channel capacity.
A cellular radiotelephone system may be initialized according to the present invention to allow simultaneous reduction of excess control channel capacity and reduction of interference with external cellular control channels. In particular, a determination is first made as to whether the common control channel can be transmitted from all the base stations over a first radio frequency without interference. If the common control cannot be transmitted from all base stations over the first radio frequency without interference, then at least one of the base stations that cannot transmit the common control channel over the first radio frequency without interference is identified.
A second radio frequency that is different from the first radio frequency is assigned for transmission by the at least one of the base stations so identified. Once initialized, the common control channel is transmitted from all the base stations except for the at least one of the base stations so identified over a first radio frequency. The common control channel from the at least one of the base stations so identified is transmitted over a second radio frequency that is different from the first radio frequency. The steps of determining, identifying and assigning may be repeatedly performed for the second radio frequency and succeeding radio frequencies until all of the base stations can transmit the common control signal without interference.
Initialization according to the invention can be particularly useful in a wireless office system wherein base stations are located in a building and may be subject to interference from other wireless office systems in the building or from a cellular radiotelephone system outside the building. This interference may be difficult to predict in advance. Accordingly, excess control channel capacity can be reduced without the need to incur interference penalties.